This invention relates generally to power switching and, more particularly, to power switching for an electric range surface heating unit.
Range surface heating units, commonly referred to as burners, typically include a microcomputer which controls energization of triacs. Specifically, the microcomputer controls the operation of triac gates, which results in energizing and de-energizing a surface heater. Over the expected life of the heating unit, the triacs may need to cycle over one million times. Triac systems capable of such cycling are expensive.
Infinite heat switches also can be used to control heat generated by a range surface burner. With such switches, the switch duty cycle is controlled to control the supply of a voltage to the burner. Controlling the switch duty cycle therefore results in controlling the energy, or heat, output of the burner. For example, at a low power setting, a duty cycle which results in voltage being supplied to the burner for 25% of the time is utilized. Such cycling results in uneven cooking in that for a selected period of time, the burner is ON, and for the remainder of the cycle, the burner is OFF. Approximately 80% of surface heater unit cooking is accomplished at intermediate duty cycles of 34% or less.
Accordingly, it would be desirable to provide a cost effective alternative to triacs for switching and controlling power to range surface heaters that produces even heating at intermediate power levels.
In an exemplary embodiment of the invention, a surface heater unit comprises an input selector, a control, a first relay switch, a second relay switch, and a surface heater. The input selector is operatively connected to the control, and the control is responsive to the input selector. The surface heater is operatively connected between the relay switches, and the switches are operatively connected to the control. The first and second switches are connected to a first power line and a second power line, respectively, and are alternately opened and closed to energize and de-energize the heater with power from one or both of the first and second power lines.
Thus, the first and second relay switches replace a single triac or infinite heat switch for controlling power to a surface heater. Consequently, the splitting of the making and breaking of current paths between two switches rather than one allows first and second relay switches to be used to increase system life. Further, because most cooking occurs at lower power levels, the first and second relay switches may be operated at current levels at about or below one-half of the rated current of the switches. Consequently, the expected life of the low cost first and second relay switches is greatly extended.
Also, the reduced power to the surface heaters that allows for the extended lives of the first and second relays switches requires a longer duty cycle to generate a given amount of heat than a comparable surface heater controlled with infinite heat switches or triacs. Thus, rather than the intense bursts of energy for a short period of time that triacs and infinite heat switches produce, the first and second relay switches supply a reduced energy level to the surface heater for a longer period of time. The temperature fluctuation of the surface heater is therefore decreased, and a steady heat is produced for a longer time period.
Thus, a long life range surface heater unit is provided that is less expensive than triacs, and that generates even surface unit heating at intermediate power levels.